"We are working diligently to identify the sender of that email at this time," police wrote in their message, which did not disclose the contents of the message.

The Franklin Township School District includes three elementary schools: Janvier School, the Main Road School and the Reutter School.

Delsea Regional High School, which is located in Franklinville, also closed for the day, according to a message on the district website. Just over 1,000 students from Elk Township and Franklin Township attend Delsea.

Two Delsea students, both under the age of 18, were charged on Monday with making terroristic threats and causing false public alarm after they were overheard making comments about school violence. Police determined no threat existed at the school and released the teens to the custody of their parents.

The districts are only the second and third to close in New Jersey because of online threats made in the two weeks since the mass shooting at Stoneman Douglas High School in Parkland, Florida.

Two students at Lakewood High School were charged on Monday after sending an email making threats about "blowing up" or "shooting up" an unspecified school. Investigators said the email came from the high school with the name of a student, but after questioning two students, police determined it was an "unamusing" hoax.

In Somerset County on Friday, a 17-year-old male student who brought a loaded 9mm semi-automatic handgun to Franklin High School was taken into custody and charged with possession of a firearm and second-degree possession of a weapon for an unlawful purpose.